a) Field of the Invention
The present invention relates generally to arrays of protein-capture agents and methods for the parallel detection and analysis of up to a large number of proteins in a sample. More specifically, the present invention relates to proteomics and the measurement of gene activity at the protein level in cells.
b) Description of Related Art
Although attempts to evaluate gene activity and to decipher biological processes including those of disease processes and drug effects have traditionally focused on genomics, proteomics offers a more direct and promising look at the biological functions of a cell. Proteomics involves the qualitative and quantitative measurement of gene activity by detecting and quantitating expression at the protein level, rather than at the messenger RNA level. Proteomics also involves the study of non-genome encoded events including the post-translational modification of proteins, interactions between proteins, and the location of proteins within the cell. The structure, function, or level of activity of the proteins expressed by a cell are also of interest. Essentially, proteomics involves the study of part or all of the status of the total protein contained within or secreted by a cell.
The study of gene expression at the protein level is important because many of the most important cellular processes are regulated by the protein status of the cell, not by the status of gene expression. Also, the protein content of a cell is highly relevant to drug discovery efforts since most drugs are designed to be active against protein targets.
Measuring the mRNA abundances of a cell provides only an indirect and incomplete assessment of the protein content of a cell. The level of active protein that is produced in a cell is often determined by factors other than the amount of mRNA produced. For instance, both protein maturation and protein degradation are actively controlled in the cell and a protein""s activity status can be regulated by post-translational modifications. Studies comparing mRNA transcript abundances to protein abundances have found only a limited correlation (coefficient of about 0.43-0.48) between the two (Anderson and Anderson, Electrophoresis, 19:1853-1861, 1998). Furthermore, the extreme lability of RNA in samples due to chemical and enzymatic degradation makes the evaluation of genetic expression at the protein level more practical than at the mRNA level.
Current technologies for the analysis of proteomes are based on a variety of protein separation techniques followed by identification of the separated proteins. The most popular method is based on 2D-gel electrophoresis followed by xe2x80x9cin-gelxe2x80x9d proteolytic digestion and mass spectroscopy. Alternatively, Edman methods may be used for the sequencing. This 2D-gel technique requires large sample sizes, is time consuming, and is currently limited in its ability to reproducibly resolve a significant fraction of the proteins expressed by a human cell. Techniques involving some large-format 2D-gels can produce gels which separate a larger number of proteins than traditional 2D-gel techniques, but reproducibility is still poor and over 95% of the spots cannot be sequenced due to limitations with respect to sensitivity of the available sequencing techniques. The electrophoretic techniques are also plagued by a bias towards proteins of high abundance.
Standard assays for the presence of an analyte in a solution, such as those commonly used for diagnostics, for instance, involve the use of an antibody which has been raised against the targeted antigen. Multianalyte assays known in the art involve the use of multiple antibodies and are directed towards assaying for multiple analytes. However, these multianalyte assays have not been directed towards assaying the total or partial protein content of a cell or cell population. Furthermore, sample sizes-required to adapt such standard antibody assay approaches to the analysis of even a fraction of the estimated 100,000 or more different proteins of a human cell and their various modified states are prohibitively large. Automation and/or miniaturization of antibody assays are required if large numbers of proteins are to be assayed simultaneously. Materials, surface coatings, and detection methods used for macroscopic immunoassays and affinity purification are not readily transferable to the formation or fabrication of miniaturized protein arrays.
Miniaturized DNA chip technologies have been developed (for example, see U.S. Pat. Nos. 5,412,087, 5,445,934, and 5,744,305) and are currently being exploited for the screening of gene expression at the mRNA level. These chips can be used to art determine which genes are expressed by different types of cells and in response to different conditions. However, DNA biochip technology is not transferable to protein-binding assays such as antibody assays because the chemistries and materials used for DNA biochips are not readily transferable to use with proteins. Nucleic acids such as DNA withstand temperatures up to 100xc2x0 C., can be dried and re-hydrated without loss of activity, and can be bound physically or chemically directly to organic adhesion layers supported by materials such as glass while maintaining their activity. In contrast, proteins such as antibodies are preferably kept hydrated and at ambient temperatures are sensitive to the physical and chemical properties of the support materials. Therefore, maintaining protein activity at the liquid-solid interface requires entirely different immobilization strategies than those used for nucleic acids. The proper orientation of the antibody or other protein at the interface is desirable to ensure accessibility of their active sites with interacting molecules. With miniaturization of the chip and decreased feature sizes, the ratio of accessible to non-accessible and the ratio of active to inactive antibodies or proteins become increasingly relevant and important.
Thus, there is a need for the ability to assay in parallel a multitude of proteins expressed by a cell or a population of :cells in an organism, including up to the total set of proteins expressed by the cell or cells.
The present invention is directed to arrays of protein-capture agents and methods of use thereof that satisfy the need to assay in parallel a multitude of proteins expressed by a cell or population of cells in an organism, including up to the total protein content of a cell.
In one embodiment, the present invention provides an array of protein-capture agents comprising: a substrate; at least one organic thinfilm covering some or all of the surface of the substrate; and a plurality of patches arranged in discrete, known regions on the portions of the substrate surface covered by organic thinfilm, wherein (i) each patch comprises protein-capture agents immobilized on the organic thinfilm, where the protein-capture agents of a given patch are capable of binding a particular expression product, or a fragment thereof, of a cell or population of cells in an organism; and (ii) the array comprises a plurality of different protein-capture agents, each of which is capable of binding a different expression product, or fragment thereof of the cell or population of cells in the organism.
In a second embodiment, the invention provides an array of bound proteins which comprises both the array of protein-capture agents of the invention and a plurality of different proteins which are expression products, or fragments thereof, of a cell or population of cells in an organism, where each of the different proteins is bound to a protein-capture agent on a separate patch of the array.
Methods of using the arrays of protein-capture agents of the invention are also provided. In one embodiment of the invention, a method of assaying in parallel for a plurality of different proteins in a sample which are expression products, or fragments thereof, of a cell or a population of cells in an organism, is provided which comprises first delivering the sample to the array of protein-capture agents of the invention under conditions suitable for protein binding, wherein each of the proteins being assayed is a binding partner of the protein-capture agent of at least one patch on the array. The final step comprises detecting, either directly or indirectly, for the presence or amount of protein bound to each patch of the array. This method optionally further comprises the step of further characterizing the proteins bound to at least one patch of the array.
In another embodiment of the invention, a method for determining the protein expression pattern of a cell or a population of cells in an organism is provided which comprises first delivering a sample containing the expression products, or fragments thereof, of the cell or population of cells to the array of protein-capture agents of the invention under conditions suitable for protein binding. The final step comprises detecting, either directly or indirectly, for the presence or amount of protein bound to each patch of the array. In an alternative embodiment, a similar method for comparing the protein expression patterns of two cells or populations of cells is also provided.
In still another embodiment of the invention, an alternative method of assaying in parallel for a plurality of different proteins in a sample which are expression products, or fragments thereof, of a cell or a population of cells in an organism is provided. The method of this embodiment comprises first contacting the sample with an array of spatially distinct patches of different protein-capture agents under conditions suitable for protein binding, wherein each of the proteins being assayed is a binding partner of the protein-capture agent of at least one patch on the array. The last step of the method involves detecting, either directly or indirectly, for the presence or amount of protein bound to each patch of the array.
In a still further embodiment, a method of producing an array of protein-capture agents is provided which comprises the following steps: selecting protein-capture agents from a library of protein-capture agents, wherein the protein-capture agents are selected by their binding affinity to the proteins from a cellular extract or body fluid; producing a plurality of purified samples of the selected protein-capture agents; and immobilizing the protein-capture agent of each different purified sample onto an organic thinfilm on a separate patch on the substrate surface to form a plurality of patches of protein-capture agents on discrete, known regions of the surface of a substrate.
In an alternative embodiment, the invention provides a method for producing an array of protein-capture agents which comprises a first step of selecting protein-capture agents from a library of protein-capture agents, wherein the protein-capture agents are selected by their binding affinity to proteins which are the expression products, or fragments thereof, of a cDNA expression library. The second step of the method comprises producing a plurality of purified samples of the protein-capture agents selected in the first step. The third step comprises immobilizing the protein-capture agent of each different purified sample onto an organic thinfilm on a separate patch on the substrate surface to form a plurality of patches of protein-capture agents on discrete, known regions of the surface of a substrate.